FlxColor: reduce color presets to 15, closes #1117

These are the colors suggested by @IQAndreas in #1117, except I decided not to add any `LIGHT_` / `DARK_`-prefixed ones because it seems inconsistent if these are not available for every color (+ there's `FlxColor#darken()` / `#lighten()`). The values are all consistent with the [Web Colors](http://en.wikipedia.org/wiki/Web_colors), except for `BROWN`.